Fix ClientOptions types

Remove index signature from ChannelOptions to fix intersection error
described in grpc#1558 which causes issues on using ClientOptions direct
fields with TypeScript.

Removing of index signature required few minor changes:
 - adding few constant types that were used throughout the app
 - using `as const` assertion in xds-client
 - using not-so-great type cast in channelOptionsEqual

Alternative solution would be removing the index signature from
ChannelOptions explicitly in ClientOptions definition, which is not
trivial and probably calls for a generic type helper.

See: grpc#1558
Fixes: grpc#1558

packages/grpc-js/src/channel-options.ts

@@ -33,7 +33,9 @@ export interface ChannelOptions {
   'grpc.max_send_message_length'?: number;
   'grpc.max_receive_message_length'?: number;
   'grpc.enable_http_proxy'?: number;
-  [key: string]: string | number | undefined;
+  'grpc.http_connect_target'?: string;
+  'grpc.http_connect_creds'?: string;
+  [key: string]: any;
 }
 
 /**

packages/grpc-js/src/xds-client.ts

@@ -746,7 +746,7 @@ export class XdsClient {
        * recursively using an xDS load balancer for the xDS client would be
        * bad */
       'grpc.service_config',
-    ];
+    ] as const;
     for (const arg of channelArgsToRemove) {
       delete channelArgs[arg];
     }